This invention relates to a method of producing tin-free steel sheets having improved retorting resistance, and more particularly, to a method of producing tin-free steel sheets exhibiting improved retorting resistance with respect to paint adhesion and suitable for use as bonded can-forming material.
Electrolytic chromate treated steel sheets also known as tin-free steel (TFS) of chromium type have improved properties as can-forming material and are regarded as a substitute for tin plates. The demand for them is increasing in these years.
Since TFS has metallic chromium and hydrated chromium oxide coatings on the surface, it does not possess sufficient weldability. A can must be fabricated from a TFS sheet by applying an exposy-phenol resin paint to a blank and bonding the mating edges of the blank with a polyamide adhesive to form a can barrel.
Recently, the extent of application of TFS cans has been further spread. That is, TFS cans are not only used for so-called cold packs prepared by packing contents such as carbonated beverage and beer in cans at relatively low temperatures, but also used for so-called hot packs prepared by packing contents such as fruit juice and coffee in cans at relatively high temperatures for sterilization. TFS is also used in those cans requiring a high temperature retorting treatment for sterilization at the end of packing. In the latter applications, there often occurred accidents of rupture of can barrels.
This can barrel rupture occurs in bonded TFS cans during hot packing and retorting treatment because hot water penetrates through the paint film at the barrel junction to deteriorate the interfacial adhesion between the paint film and the TFS substrate to eventually separate the paint film from the TFS substrate.
Research works revealed that sulfuric acid, which was conventionally added to chromium plating baths and electrolytic chromate baths, was codeposited in the hydrated chromium oxide coating and the sulfuric acid codeposited was dissolved out during the subsequent retorting treatment to give rise to the paint film-TFS substrate interfacial separation. Several proposals were made to avoid sulfuric acid codeposition, for example, by using sulfuric acid-free plating baths, or by excluding sulfuric acid from acid pickling solutions used in a pre-treatment. However, these techniques had a number of industrial problems in that manufacture efficiency is considerably lowered, product quality is less consistent, and yield is low as compared with the traditional techniques.
It is, therefore, an object of the present invention to eliminate the above-mentioned problems of the prior art and to provide an improved method of producing tin-free steel sheets which do not undergo any interfacial separation between a TFS substrate and a paint build-up during a retorting treatment.
The inventors previously proposed in Japanese Patent Application No. SHO-56-62766 a technique capable of overcoming the problem of sulfuric acid codeposition wherein chromium plating is followed by a reverse electrolysis treatment of anodizing the plated steel sheet in the chromium plating solution, and then by an electrolytic chromate treatment in an aqueous chromate solution.
The electrolytic chromate treating solution may desirably contain sulfate ions as little as possible. However, commercially available chromates (CrO.sub.3) contain sulfate as one of impurities. It is thus imperative that electrolytic chromate solutions prepared therefrom contain the sulfate contaminant. An electrolytic chromate solution whose sulfate concentration is as low as 0.005 g/l will be prepared from a reagent grade chromate which is too expensive to gain commercial acceptance. The use of commercial grade chromates results in electrolytic chromate solutions, some of which are successful in improving retorting resistance while the remainings fail.
Making researches how the retorting resistance depends upon the concentration of SO.sub.4.sup.-- in the chromate solution used in the electrolytic chromate treatment after the reverse electrolysis, the inventors have found that improved retorting resistance is obtained provided that the concentration of SO.sub.4.sup.-- in the electrolytic chromate solution is limited within a certain range.